1. Field of the Invention
This invention relates to a stage apparatus for correcting the inclination of a body, and particularly to a tilting apparatus capable of supporting thereon and inclining a photosensitive substrate to which the pattern of a mask is transferred, which apparatus is suitable for an exposure apparatus used in the photolithography process for manufacturing, for example, semiconductive elements, liquid crystal display elements, thin film magnetic heads, etc.
2. Related Background Art
In an exposure apparatus used in the photolithography process, the pattern of a mask or a reticle is transferred onto a photosensitive substrate (a wafer to which photoresist is applied or a glass plate or the like). In this case, the work of setting the whole or a local area of the surface of the photosensitive substrate in parallelism to a plane which is the reference, i.e., so-called levelling (tilting), is effected. For example, in the proximity system wherein a mask and a photosensitive substrate are made proximate to each other with a gap of the order of 10 .mu.m to 300 .mu.m, the pattern surface of the mask and the surface of the photosensitive substrate are made parallel to each other over the whole area thereof. Also, in the projection system wherein the image of the pattern of a mask or a reticle is imaged on a photosensitive substrate through a projection optical system, the image plane of the projection optical system and the surface of the photosensitive substrate are made parallel to each other.
Such levelling work is done to obtain a good quality of transfer image (the quality of pattern profile formed on the photosensitive substrate) on the whole area of the surface of the photosensitive substrate to which the pattern of the mask or reticle is transferred. Particularly, in an exposure apparatus of the projection type which is provided with a projection optical system of high numerical aperture and high resolving power, the depth of focus is markedly small as compared with the size of the projection field (image field) of the projection optical system. This depth of focus .DELTA.F is represented as .DELTA.F=.lambda./(2NA.sup.2) by the wavelength .lambda. of illuminating light for exposure and the numerical aperture NA of the projection optical system.
Assuming here that .lambda.=365 [nm] (the i-line of a mercury lamp) and NA=0.6, the actually effective depth of focus .DELTA.F is about 0.5 .mu.m (1 .mu.m in terms of the width in the direction of the optical axis of the projection optical system) relative to the best focus position. In contrast, the size of the pattern image projected differs depending on the exposure apparatus, but is of the order of 15 mm.times.15 mm in a typical wafer stepper. This, when considered in terms of the thickness of a plate material of 1 m square, corresponds to only 66.7 .mu.m. Because of such very small depth of focus, in a projection exposure apparatus, it is necessary to effect levelling accurately with the localized waviness or the like of the surface of the photosensitive substrate taken into account to thereby keep the degree of parallelism thereof to the projection image plane.
To effect such levelling, it is necessary to accurately measure the amount of inclination of the whole or a local area of the surface of the photosensitive substrate from a fiducial plane. Various measuring systems therefor have heretofore been proposed, and as typical ones of them, there are known (A) Japanese Patent Application Laid-Open No. 58-103136, (B) U.S. Pat. No. 4,084,903, (C) U.S. Pat. No. 4,558,949 and (D) U.S. Pat. No. 4,383,757.
In publication (A), there is disclosed a system in which the height position of each of a plurality of points (three or more points) on a photosensitive substrate is measured by a gap sensor such as an air micrometer and on the basis of the measured values and the coordinates value of each measuring point in XY plane, the approximate plane expression of the surface of the photosensitive substrate is specified by the least square method, whereafter the amount of deviation of the approximate plane from a fiducial plane is determined by the coordinates positions of the three drive points of a levelling mechanism for the photosensitive substrate, and the amount of deviation in the height direction at each of those drive points is corrected.
U.S. patent (B) discloses a system in which the nozzles of an air micrometer are disposed at four locations around the lower portion of the barrel of a projection optical system and two of those four nozzles are provided on the X-axis and the remaining two are provided on the Y-axis, and the distance (gap) between the surface of a photosensitive substrate and the barrel is measured by each nozzle, and the amount of inclination .alpha..sub.y of the photosensitive substrate about the Y-axis is found from the difference in back pressure between the two nozzles on the X-axis and the amount of inclination .alpha..sub.x of the photosensitive substrate about the X-axis is found from the difference in back pressure between the two nozzles on the Y-axis, and by the use of these amounts of inclination .alpha..sub.x and .alpha..sub.y, a holder for the photosensitive substrate is vertically moved by three piezo elements to thereby correct the inclination.
In U.S. patent (C), there is disclosed a system in which collimated light is applied from an oblique direction to a local area on a photosensitive substrate to which a projected image by a projection optical system is transferred, and the reflected light thereof (a parallel beam of light) is imaged into a spot-like shape by a condensing lens and is received by a four-division photoelectric element, and the average amount of inclination of the local area on the photosensitive substrate is detected from a variation in the received position of the spotlight on the four-division photoelectric element.
U.S. patent (D) discloses a system in which a light spot is projected onto each of a plurality of points on a photosensitive substrate through a projection optical system, and the reflected light thereof is re-imaged through the projection optical system and from a variation in the contrast of this re-imaged image, the focus error at the projected point of each light spot on the photosensitive substrate in the direction of the optical axis of the projection optical system (Z-direction) is detected, and a holder for holding the photosensitive substrate is inclined by three drive units (servo-motors) so that the focus error may be substantially zero at each projected point.
Besides the above-described conventional techniques (A) to (D), what is necessary for levelling is the structural precision and stability of the levelling mechanism itself, and a conventional technique therefor is disclosed in U.S. patent (D). As the levelling mechanism, there is also known a system as disclosed in (E) U.S. Pat. No. 4,770,531 wherein drive points in Z-direction are provided at locations trisecting the circumference of a levelling table and at each drive point, the levelling table and the base thereof are coupled together by a doughnut-shaped leaf spring to thereby enhance the lateral rigidity of the levelling table.
There is further known in (F) U.S. Pat. No. 4,504,144 a levelling control system in which a focus error is measured at each of three points in a local area on a photosensitive substrate to which the pattern image of a mask or a reticle is transferred, and from the result of the measurement, the amounts of inclination (.theta.x, .theta.y) in the direction of the X-axis and the direction of the Y-axis in a field and the overall focus error amount (f) are found as analog values and by the use of the coordinates position (analog value) of a field on a wafer and the coordinates positions (analog values) of three drive points on a wafer levelling table, besides the amounts of inclination (.theta.x, .theta.y) and the focus error amount (f), the amount of movement of a servo-motor at each of the three drive points (the amount of correction of the drive point in Z-direction) is calculated by an analog calculation circuit.
Among the conventional techniques as described above, the levelling control system disclosed in publication (A) has suffered from the inconvenience that the heights at three or more measuring points on the photosensitive substrate are measured by the gap sensor such as an air micrometer and therefore the time required for the measurement of the heights is long and the responsiveness of the levelling operation is bad. In a control system wherein the responsiveness of the levelling operation is bad, it is difficult to apply to an exposure apparatus a scanning system particularly such as a step and scan system in which a reticle and a photosensitive substrate are synchronously scanned to thereby successively transfer the pattern images of the reticle onto the photosensitive substrate.
The levelling control system disclosed in U.S. patent (B), which uses an air micrometer, is similar to publication (A) in that the responsiveness is bad. Further, in U.S. patent (B), the angles of inclination about two axes are first calculated from the focus positions at four points and the amount of drive of the driving element is newly calculated from those angles of inclination, and this has led to the inconvenience that the calculation time becomes longer and the responsiveness becomes worse.
Also, in U.S. patent (C), only the average angle of inclination is measured and the focus position (the height) is not measured, and this has led to the inconvenience that a sensor for the detection of the focus position becomes discretely necessary and the control mechanism is complicated. In U.S. patent (D), the focus position at each point on the photosensitive substrate is measured through the projection optical system, and this leads to the possibility that this system cannot be applied to an exposure apparatus which does not use a projection optical system and that an illumination optical system or the like for applying illuminating light to the reticle becomes complicated. On the other hand, the system in which the lateral rigidity is enhanced as disclosed in U.S. patent (E) suffers from no special inconvenience.
Further, in the levelling control system disclosed in U.S. patent (F), as in U.S. patent (B), two steps of calculation in which the angles of inclination about two axes and the focus position are first calculated from the focus positions at three points and the amount of drive of the driving element is newly calculated from those angles of inclination and the focus position are effected, and this has led to the inconvenience that the calculation time becomes longer and the responsiveness becomes worse. Also, only the focus positions at three points are measured, and this leads to the inconvenience that when for example, there is a measuring point which is peculiarly high or low in the focus position, the errors of the angle of inclination of the exposed surface of the photosensitive substrate with respect to the fiducial plane and the focus position become great.
In this connection, when the pattern images of the reticle are to be transferred to each shot area of the photosensitive substrate, unevenness (level difference) is sometimes created in each shot area by the process hitherto. Patterns of various line widths are mixedly included in the pattern images of the reticle and therefore, to form a clear-cut image on the whole surface of each shot area, it is desirable that the area on the photosensitive substrate to which one of the pattern images of the reticle which is narrowest in line width is transferred be adjusted to the fiducial plane. In the conventional levelling control system, however, it has been impossible to preponderantly adjust a desired area on the photosensitive substrate to the fiducial plane.